In an existing mobile communication system, such as high-speed downlink packet access (HSDPA: High Speed Downlink Packet Access), an adaptive modulation and a channel coding (AMC: Adaptive Modulation and Channel coding) are performed in order to improve a data throughput (particularly, a data throughput in a downlink).
In the AMC, the modulation method and the channel coding ratio are appropriately changed (in every TTI (Transmission Time Interval) of about 0.5 ms in an extreme example) depending on the quality of a channel condition. Accordingly, the data can be transmitted by a larger amount and at a higher speed.
In particular, the AMC can greatly improve the throughput when data having a long packet length is transmitted.
In the AMC, it is required to notify, to a user equipment, an appropriate modulation method and a channel coding ratio for a downlink data channel (shared data channel), in each downlink data transmissions. Such notification is performed by utilizing a downlink control channel that is referred to as an L1/L2 control channel (L1/L2 signaling channel).
The L1/L2 control channel includes information essential for decoding the downlink data on the downlink data channel (may include not only information relating to the AMC, but also information relating to a frequency resource block used for the downlink data transmission or the like). For this reason, the L1/L2 control channel has to be transmitted along with the downlink data channel, for every downlink data channel transmission.
Accordingly, when downlink data having a short packet length is frequently transmitted, a ratio of a radio resource assigned to the L1/L2 control channel (downlink control channel) is increased while a radio resource assigned to the downlink data channel is decreased. This is because the L1/L2 control channel has to be transmitted while being associated by all the downlink data channels each transmitting the downlink data.
Here, representative examples of data generated frequently and having a short packet length include a voice packet, a packet for VoIP, and real time data.
In order to cope with such an inconvenience, a technique referred to as “Persistent Scheduling” has been proposed.
In this technique, downlink data (typically, a voice packet) is transmitted via a downlink data channel by one fixed transmission format in a predetermined cyclic period, for example, 20 ms.
Such a transmission format includes information necessary for decoding the downlink data on the downlink data channel, such as the modulation method and the channel coding ratio.
For example, such technique is configured so that the modulation method is fixedly set to “QPSK,” that the channel coding ratio is fixedly set to “1/3,” and that the transmission format including the modulation method and the channel coding ratio is known to a base station and a user equipment.
Accordingly, the user equipment can properly receive the downlink data on the downlink data channel without receiving the downlink control channel such as the L1/L2 control channel.
It is also proposed that two types of transmission formats are prepared as the aforementioned transmission format. In this case, the L1/L2 control channel is not used either.
In this case, the user equipment tries to decode the downlink data on the downlink data channel by using both of the two types of the transmission formats. Then, the downlink data having been successfully decoded will be used in further processing in a subsequent stage. Such a technique is also referred to as “Blind Detection.”
In any case, the persistent scheduling deals with the above-mentioned problem by decreasing options of the transmission format of the downlink data channel that transmits the downlink data such as a voice packet, thereby omitting the L1/L2 control channel.
Note that, such persistent scheduling is described in, for example, non-patent document 1 and non-patent document 2.
Further, in the mobile communication system based on an OFDM (Orthogonal Frequency Division Multiplexing) technique, it has also been known that performing a power balance control (transmission power control) at a sub-carrier level in addition to the AMC is effective in order to improve the utilization efficiency of the frequency (for example, see non-patent document 3 and non-patent document 4).
Furthermore, as a method for indirectly notifying a propagation loss in the downlink in order to perform the transmission power control, performing a transmission power control by using a CQI is known. Applying such method to an L1/L2 control channel has also been proposed (for example, see non-patent document 5 and non-patent document 6).
However, in the above-described technique, the number of options of the transmission format of the downlink data channel is considerably limited (is limited to approximately the number making the blind detection executable). Accordingly, the above-described technique has a disadvantage from the viewpoint of improving the downlink data transmission efficiency in consideration of the channel condition.